418 MR. DUGALD M‘KICHLAN ON THE DETERMINATION OF THE NUMBER 
The moment of inertia of the vibrator with which the coil was compared was 4307*498 
centimetre-grammes. 
The periods of vibration of coil and vibrator similarly suspended were 4*69 and 7*795 
seconds respectively. Whence for the coil 
W& 2 = 4307*498 X ( 7 4 - 7 QA ) 2 = 1559*3 centimetre-grammes. 
Addition for cross piece = 0*2 „ „ 
Total moment of inertia =1559*5 ,, „ 
Period of vibration of coil after being suspended in the electrodynamometer was 
T = 4*800 seconds. 
Distance of scale= 446*5 centims. 
Length of each scale-division inch = 2 ’ 5 ^ 9 9 - 5 - 4 centims. 
Number of turns on left-hand coil, %=3720*75. 
Number of turns on right-hand coil, n' = 3668*50. 
Mean radius of left-hand coil, r=16*ll centims. 
Mean radius of right-hand coil, r'= 16*24 centims. 
Distance of mean plane of coils from centre of the suspended coil, Z> = 8*185 centims. 
«Vc 2 (determined by experiment May 10, 1872)=195,420 sq. centims. 
These values being substituted in equation (5), for this series of observations 
y= *000021842 electromagnetic units. 
The following Tables exhibit results obtained from simultaneous measurements of 
current and electromotive force. It would have been desirable to have had more frequent 
measurements of the resistance of the coils ; but as only two observers were generally 
available, and the comparison of the electrodynamometer and electrometer required the 
attention of two observers, this was not generally practicable. There was always a 
liability in the coils to a variation in resistance, due to the variation in the temperature 
of the coils caused by the current ; but as the current was not allowed to pass through 
the coils for a longer time than was just necessary for its measurement, these variations 
were not considerable. 
The plan of the connexions is indicated in Plate XXXII. fig. 4. Two terminals were 
led from the battery (B) to a reversing key (K). The wires leading from the key were 
connected one to the end of the left-hand coil (A'), the other to the opposite end of the 
right-hand coil (A). From these points, where the battery joined the electrodynamo- 
meter coils, electrodes were brought to another reversing key (C) which was connected 
with the absolute electrometer (E), one of its wires being connected with the insulated 
electrode of the electrometer which communicated with the insulated lower plate, the 
other being in contact with the outside coating of the jar of the electrometer. The 
difference of potentials thus measured was not the entire difference of potentials between 
the two poles of the battery, but the reduced difference maintained between the extre- 
mities of the electrodynamometer circuit. 
When two deflections, with the current passing in opposite directions, had been read 
